Method and an arrangement for the manufacture of casings

ABSTRACT

In the packaging field certain types of packages are manufactured from pre-shaped, cylindrical casings made from a heat-shrinkable laminate. The casings are formed by winding a material sheet around a mandrel and subsequent sealing together of the overlapping ends of the sheet. However, because of the stiffness of the material the leading end of the sheet protrudes tangentially from the mandrel during the winding and prevents an accurate adaptation of the material sheet to the surface of the mandrel. 
     According to the method and arrangement of this invention this disadvantage is avoided in that the leading end of the sheet is pre-heated on the side facing the mandrel to such a degree, that the shrinking is initialled. The shrinking of one side only results in that the leading end of the material sheet is given a curved shape with a radius that coincides with the radius of the mandrel, and consequently the leading end is no longer an obstacle to the winding.

This is a division of Application Ser. No. 972,802, filed Dec. 26, 1978now U.S. Pat. No. 4,284,448.

The present invention relates to a method in the manufacture of casingsof thermoplastic, heat-shrinkable material by winding of a materialsheet around a forming tool and joining together the ends of the sheet.

The invention relates also to an arrangement for the realization of themethod, which arrangement comprises a forming tool for the winding of amaterial sheet to the shape of a casing and elements for the joiningtogether of the ends of the material sheet in the shape of a casing.

Casings of thermoplastic, heat-shrinkable material are frequentlymanufactured as an intermediate or semifinished product in themanufacture of packing containers, drinking cups, protective casingsetc. The casings are most frequently of cylindrical or conical shape andare intended to be given in a subsequent shrinkage the desired finalshape which is adapted to the final area of application.

In the manufacture of a packing container of e.g. the type which isdescribed in Swedish Pat. No. 381,442, to which reference is made, asheet of laminated material is used comprising a central layer of foamedplastic which is covered on both sides by homogeneous thermoplasticlayers. The laminate has been treated during manufacture in order togive it heat-shrinking properties, that is to say, the material willshrink when it is heated above a certain temperature limit.

The laminate sheet is wound around a forming tool in the shape of arotatable, substantially cylindrical mandrel so that the two ends of thesheet will somewhat overlap one another, whereupon the ends are made tolie against one another and are sealed together so that a casing with alongitudinal overlap joint is formed. The ready-shaped casing is thenmoved to a profiled mandrel end where the casing is placed so that witha part of its length it extends beyond the end of the mandrel. A plateof thermoplastic material, e.g. of the same material as the casing, isthen placed against the end surface of the mandrel, whereupon thematerial is heated up above its softening temperature so that the freeend of the casing shrinks and adapts itself to the shape of the mandrel.By complementary heating the free edge surface of the casing is softenedso that when subsequently pressure is exercised it is heat-sealed to theedge area of the material plate. The forming of the container body isthen complete.

When during the course of manufacture described the material sheet is tobe wound around the forming tool, the material sheet is fed to theforming tool mainly tangentially, whereupon the front end of thematerial sheet is retained mechanically against the shell surface of theforming tool and the forming tool is made to rotate through approx.360°. The mechanical retaining device provided on the forming tool is inthe form of a small finger which extends axially in relation to theforming tool and is movable between two positions, namely an openposition at a distance from the shell surface and a closed position incontact with the shell surface. When a material sheet is introduced, itsfront end is fed into the space between the retaining finger in its openposition and the shell surface, whereupon the finger is moved to theclosed position so that the front end of the material sheet is held inplace tightly. To make possible the subsequent sealing of the ends ofthe wound material sheet it is essential that the material sheet beforethe clamping of its front end should be advanced to such an extent thatthe clamping finger engages at a certain distance inside the materialsheet so that a free end is available which is sufficiently large toallow the formation of a longitudinal joint after the winding has beencompleted. After clamping between the clamping finger and the shellsurface the free end will extend tangentially from the shell surface ofthe forming device. When the material sheet after rotation of theforming tool has been wound up around the forming tool, the free frontend of the sheet prevents the rear end of the sheet from making contactwith the shell surface of the forming tool, since the rear end firstmeets and comes to rest against the front edge of the sheet situated ata distance from the shell surface. When after the heating of the twoends they are pressed together for the forming of the heat-sealedoverlap joint, a material surplus exists owing to the unsatisfactorycontact of the sheet with the shell surface thus causing the diameter ofthe casing to increase a little after the sealing which is adisadvantage during the further forming.

The problem described is particularly noticeable when the casings aremade of rigid and thick material, e.g. a polystyrene laminate comprisinga central layer of foamed polystyrene, which is comparatively thick andis covered on both sides by thin homogeneous layers.

Earlier efforts to solve the problem were concentrated first andforemost on the design of the retaining finger and the area of the shellsurface of the forming tool co-operating with the retaining finger withthe object of seeking by mechanical means to cause the free materialends to adapt themselves more accurately to the shape of the shell.These efforts, however, met with little success.

It is an object of the present invention to eliminate theabove-mentioned disadvantage and to make possible an accurate adaptationof the front end of the material sheet to the shell surface alreadybefore the winding of the material sheet around the forming tool.

This object has been achieved in accordance with the invention in that amethod for the manufacture of casings from a thermoplastic,heat-shrinkable material by winding of a material sheet around a formingtool and joining together of the ends of the sheet, has been given thecharacteristic that the material sheet before winding around the formingtool is preheated along its whole length at least on the side facing theforming tool to a temperature which substantially coincides with thesoftening temperature of the material and that the front end of thematerial sheet is subjected on the side facing towards the tool to acomplementary intensive heating to a temperature which exceeds thetemperature at which shrinkage is initiated, so that owing to theshrinkage capacity of the material a curved shape in the directiontowards the tool is imparted to the said sheet end. By complementing thesoftening preheating of the material with an intensive heating of alimited surface area at the front end of the sheet it becomes possible,without mechanical work upon the material sheet to cause this end to bein contact with the shell surface of the forming tool.

An embodiment of the method in accordance with the invention has beengiven the further characteristic that in case of a substantiallycylindrical tool the front end of the sheet is given a curvature, theradius of which largely coincides with the radius of the forming tool.

Another embodiment of the method in accordance with the invention hasbeen given the further characteristic that in the laminated materialonly the material layer or layers closest to the forming tool is/areheated in the intensive heating phase.

It is another object of the present invention to provide a suitablearrangement for the realization of the method in accordance with theinvention, which arrangement in a space and energy saving manner makespossible the realization of the method in accordance with the invention.

It is a further object of the present invention to provide anarrangement in which a single stationary heating unit complies with theheating needs which arise in connection with the winding of the sheetand the forming of the casing.

These and other objects have been achieved in accordance with theinvention in that an arrangement for the realization of the method inaccordance with the invention comprising a forming tool for the windingof a material sheet to casing-shape and elements for the joiningtogether of the ends of the material sheet in casing-shape has beengiven the characteristic that the heating set is arranged along a feedtrack for the material sheet, which set comprises a heating unit for theheating of the main length of the material sheet and an intensiveheating unit for the heating of the side of the front end of the sheetwhich faces towards the forming tool.

An embodiment of the arrangement in accordance with the invention hasbeen given the further characteristic that the intensive heating deviceis so placed adjoining the forming tool that before the winding of thematerial sheet around the forming tool it intensively heats the side ofthe front end of the sheet facing towards the tool and after the windingof the sheet it heats the ends of the sheet so as to make possible theirheat-sealing.

A further embodiment of the arrangement in accordance with the inventionhas been given the further characteristic that the heating set isstationary.

Another embodiment of the arrangement in accordance with the inventionhas been given the further characteristic that the preheating unit aswell as the intensive heating unit has a working width whichsubstantially coincides with or slightly exceeds the corresponding widthof the material sheet, that the preheating unit and the intensiveheating unit together have a working length corresponding to the lengthof the material sheet and that the working length of the intensiveheating unit is smaller by 1/3rd than the working length of thepreheating unit.

Another preferred embodiment of the arrangement in accordance with theinvention has been given the further characteristic that the heatingunit is placed between the preheating unit and the forming tool.

Another preferred embodiment of the arrangement in accordance with theinvention has been given the further characteristic that the heatingunit is of the hot-air type and comprises a valve for conducting the hotair to the preheating unit and/or the intensive heating unit.

Another preferred embodiment of the arrangement in accordance with theinvention has finally been given the characteristic that the valve hasthree working positions, namely a first position wherein the supply ofhot air to the heating units is interrupted, a second position whereinhot air is supplied to both heating units, and a third position whereinhot air is supplied to the intensive heating unit.

The invention will now be described in more detail with specialreference to the enclosed schematic drawings which only show the partsnecessary for the understanding of the invention.

FIGS. 1 a-e shows in successive steps the conversion of a material sheetto casing-shape in accordance with the method according to theinvention, and

FIG. 2 is a section through a preferred embodiment of a heating set inaccordance with the invention.

FIGS. 1a-1e show essential machine parts for the manufacture of casingsfrom thermoplastic, heat-shrinkable material. More particularly each ofthe figures shows a forming tool 1 which is constituted of asubstantially cylindrical mandrel. The forming tool 1 is provided with aretaining finger 2 which is movable between a position at a distancefrom the forming tool (FIGS. 1a, b, c) and a position where it restsagainst the forming tool (FIGS. 1d and e).

A material sheet 3 is fed to the space between the forming tool 1 andthe retaining finger 2 along a track extending tangentially in relationto the forming tool. Alongside this track there is a heating set 4 whichin the preferred embodiment described here consists of a hot-air setwith a preheating unit 5 and an intensive heating unit 6. Both thepreheating unit 5 as well as the intensive heating unit 6 in accordancewith the invention are constituted of arrangements of hot-air nozzleswhich are directed towards the said feed track of the material sheet.The heating set 4 is described in more detail with special reference toFIG. 2.

In the manufacture of a casing of thermoplastic, heat-shrinkablematerial in accordance with the method according to the invention, amaterial sheet 3 is fed along the said track tangentially in directiontowards the forming tool 1. The material sheet 3 has a length which isadapted so that after winding around the forming tool 1 a sufficientlywide overlap area is formed to make a sealing possible. The direction offeed of the material sheet 3 is indicated in FIG. 1a by means of anarrow. In the position shown the forming tool 1 is stationary, theretaining finger 2 is at a distance from the surface of the formingdevice and both nozzles 5 and 6 of the heating set 4 are inactive.

FIG. 1b shows an instant during the formation of the casing when thematerial sheet momentarily stops in a position just opposite the heatingset 4. The forming tool 1 is stationary and the retaining finger 2 is inits open position. Hot air is supplied to the heating set 4 and isdistributed between the preheating unit 5 and the intensive heating unit6 so that the material sheet 3 is heated along its whole length on theside facing towards the heating set, which coincides with the side ofthe sheet which in the subsequent winding faces the mandrel. On theassumption that the material sheet consists of the material combinationmentioned earlier, that is to say a comparatively thick (approx. 1 mm)central layer of foamed polystyrene which is covered on both sides bythin layers (0.1-0.2 mm) of homogeneous polystyrene, it is heated untilthe homogeneous layer facing towards the heating set attains atemperature of 100°-110° C. which is a little below the softeningtemperature of the material.

FIG. 1c corresponds to FIG. 1b in everything except the supply of hotair. The heating of the whole length of the material sheet 3 has nowbeen interrupted and the total amount of hot air is conducted towardsthe material sheet via the intensive heating nozzle 6. When the materialsheet has stopped in the position shown, the intensive heating nozzle isdirected towards the material sheet at a little distance inside thefront end of the material sheet, and the corresponding area is nowheated to a temperature exceeding that at which shrinkage is initiated,that is to say, a temperature of between 120° and 130° C. This causesthe heated area of the side of the material facing the nozzle to shrinktogether so that the front end of the material curves in the directiontowards the forming tool 1. When the heating has continued sufficientlylong for the front end of the material sheet to have a radius ofcurvature which substantially coincides with the radius of thecylindrical forming tool, the intensive heating is interrupted and thehot air supply is divided again between the preheating unit 5 and theintensive heating unit 6 in a manner corresponding to that described inconnection with FIG. 1b. At the same time the feed of the material sheet3 is restarted so that the front, curved end of the same is moved inbetween the forming device 1 and the retaining finger 2 and is held inplace in such a position that a free end extends past the finger alongthe shell surface of the forming device 1.

In FIG. 1d is shown how the material sheet 3 during continued supply ofhot air via the preheating unit 5 as well as the intensive heating unit6 commences to be wound around the forming tool which now commences torotate in the direction of the arrow. When the material sheet during thewinding passes the heating set, a further increase of the temperature ofthe side facing the set takes place so that this temperature reaches thesoftening temperature of the material, approx. 110°-120° C., whichfacilitates the forming of the material sheet to casing-shape and makespossible a winding in accurate adaptation to the shell surface of theforming tool 1.

In FIG. 1e it is shown how the forming device 1 after completed windingup of the material sheet 3 is stationary again, the rear end of thesheet overlapping the front end of the sheet and being situated rightopposite a pressure device indicated by means of dash-dotted lines. Thesupply of hot air to the heating set has again been adjusted and thetotal air stream now passes to the intensive heating unit which owing toits location introduces hot air into the pocket formed between the frontend and the rear end of the material sheet. As a result the materialsurfaces facing one another are heated to a temperature exceeding themelting temperature, approx. 150°-160° C., of the material. When thematerial after a short time has reached this temperature the pressuredevice is actuated and presses together the two ends of the materialsheet so that a material casing is formed, the inside diameter of whichcoincides accurately with the outside diameter of the forming tool 1. Atthe same time the feed of the next material sheet 3 commences and thecycle is repeated in the abovementioned manner with a cycle time ofapprox. 2 seconds.

Practical tests have shown that the method in accordance with theintention allows a repeated manufacture of casings with a diameter whichvirtually does not vary at all. This is possible primarily thanks to thebending-in of the front end of the material sheet described, since thisbending-in makes possible an accurate adaptation of the material sheetto the mandrel around the whole shell surface. The adaptation is furtherpromoted by the preheating of the side of the material sheet facing themandrel to the softening temperature of the material. If the preheatingis eliminated or if the surface of the material facing the forming toolis not heated to a sufficient degree, that is to say, the region of thesoftening temperature of the material, the layer of the material facingtowards the forming tool will on winding to casing-shape produce a greatnumber of cracks or crease lines extending axially in relation to theforming tool so that first of all the inside of the casing will not becylindrical and secondly it will have unsatisfactory dimensionalaccuracy.

The method described is particularly suitable in the manufacture ofcasings of laminated material which comprises a centrally situated,relatively thick layer of foamed plastic, since such material crackseasily when it is subjected to bending stresses above a certain limit.With this type of material it is also very simple to dose the amount ofheat supplied so that the required bending of the front end of thematerial sheet is obtained since the foamed plastic layer situatedcentrally in the material sheet acts as an insulating layer and preventsthe heat from spreading to the opposite surface layer of the materialsheet. As a result it will be simple during the intensive heating tolimit the heating to the material layer or layers which during windingwill be closest to the forming tool, and this ensures a readilycontrollable heating and hence a simple controlling of the bending sothat the radius of curvature can be made to coincide with the radius ofthe forming tool.

As described, the bending of the front end of the material sheet takesplace completely without any mechanical work upon the material sheetwhich is a great advantage since the foamed plastics used can easily bedeformed when they are subjected simultaneously to heating and tomechanical working. The absence of a mechanical forming device is anadvantage also purely practically, since such a device is quite bulkyand difficult to place in the area around the rotatable forming toolwhich area in practice is also provided with further devices for theguiding of the material sheet and the forming of the casing, whichdevices, however, are not shown, since they are well-known to thoseversed in the art and are of no importance for the understanding of theinvention.

To allow the realization of the method in accordance with the inventionit has proved appropriate to design a special arrangement for theheating of the material sheet. A preferred embodiment of thisarrangement will now be described with special reference to FIG. 2.

The arrangement, as mentioned earlier, comprises a heating set 4 whichis situated close to the forming tool 1 and comprises the two heatingunits 5 and 6. The heating units consist of hot-air nozzles which arelocated in the direct vicinity and which are pointed towards the trackalong which the material sheets are fed to the forming tool. Both thepreheating unit 5 and the intensive heating unit 6 have a working widthwhich corresponds to or slightly exceeds the corresponding width of thematerial sheet. The working length of the preheating unit 5 is slightlyless than the length of the material sheet whilst the intensive heatingunit has an appreciably shorter working length which is 1/3rd less thanthe length of the preheating unit. The preheating unit and the intensiveheating unit together however, have a working length corresponding tothe length of the material sheet. The terms "working width" and "workinglength" are here intended to mean the width and length respectively ofthe surface of the stationary material sheet which is heated by theheating unit, that is to say, which is heated to at least the desiredtemperature.

The outlet of the intensive heating unit has preferably the form of agap whose longitudinal direction is parallel with the axial center lineof the forming tool. The intensive heating unit moreover is directed sothat hot air can be blown obliquely forwards against the forming tool.

The air current is generated by means of a fan (not shown) and flows inthe direction of the arrow 7 through a heat exchanger 8, through aheating chamber 9, with heating elements in the form of electricresistance wires 10, and past a valve set 11 which distributes the airbetween a first duct 15, which is connected to the heating unit 5, asecond duct 16 which is connected to the intensive heating unit 6, and athird duct 17, which conducts the air via the heat exchanger 8 toatmosphere.

The valve set 11 comprises a valve spindle 12 which via openings inintermediate walls arranged between the different air ducts extendsthrough all the air ducts in the following order (from the top downwardsin FIG. 2): outlet duct 17, supply duct, outlet duct 16 and outlet duct15. The valve spindle 12, which is axially movable between threepositions, carries a disc valve 13 which is fixed firmly to the spindle12, and a valve disc 14 which is axially displaceable in relation to thespindle. The movement of this valve disc 14 in relation to the spindleis limited in one direction (downwards in FIG. 2) by a stop 18 on thespindle 12. The valve discs 13, 14 are adapted so as to co-operate withvalve seats 19-21 which form the previously mentioned openings in thewalls between the different air ducts. The valve seat 19 is situatedbetween the supply duct and the third outlet duct 17 and is adapted soas to co-operate with the valve disc 14 which is axially displaceable inrelation to the spindle whilst the valve seats 20 and 21 are situatedbetween the supply duct and the second outlet duct 16 and between thesecond outlet duct 16 and the first outlet duct 15 respectively, and areadapted so as to co-operate with the valve disc 13 which is firmly fixedto the spindle.

The valve set has three working positions, namely a first positionwherein the supply of hot air to the heating units is interrupted, asecond position wherein hot air is supplied to both heating units and athird position wherein hot air is supplied to the intensive heatingunit. In the first position the valve spindle 12 is in its topmost endposition when the fixed valve disc 13 rests against the valve seat 20and prevents supply of hot air to the heating units. The valve disc 14has been lifted via the stop on the valve spindle to an upper openposition and the hot air can flow out freely through the third outletduct 17, via the heat exchanger 8 and out to atmosphere. The valvepositions described are shown by means of dash-dotted lines. The valveset 11 occupies the first working position when material is being fed,that is to say, in the position which corresponds to FIG. 1a.

In the second position of the valve set 11 the valve spindle 12 is in anintermediate position and allows a flow of hot air to the intensiveheating unit as well as to the preheating unit. In this position thevalve spindle has been lowered so that the valve disc 13 is just halfwaybetween the two valve seats 20 and 21 whilst the valve disc 14 restsagainst the valve seat 19 and prevents hot air from flowing out throughthe third outlet duct 17. In this position the valve discs are shown infully drawn lines and it can be seen from the drawing that the valvedisc 13 does not rest against the valve seat 20 or the valve seat 21 sothat the air stream from the inlet duct can pass to the preheating unit5 as well as to the intensive heating unit 6. This working position isassumed by the valve set during the heating of the stationary materialsheet (FIG. 1b) and during the winding phase (FIG. 1d).

In the third working position of the valve set the total stream of hotair is supplied to the intensive heating unit 6. In this position thevalve spindle is in a lower end position with the valve disc 13 beingmoved to engage the valve seat 21 which is shown in broken lines in FIG.2. The movable valve disc 14, owing to its axial displaceability inrelation to the valve spindle 12, remains in engagement with the valveseat 19. As a result of the valve disc 13 resting against the valve seat21 the flow of hot air to the preheating unit 5 is hindered and thetotal hot air stream flows instead from the inlet duct through the valveseat 20 and via the second outlet duct 16 to the intensive heating unit6. This working position is assumed by the valve on the one hand whenintensive heating is applied to the front end of the sheet (FIG. 1c) andon the other hand during intensive heating of the two overlapping sheetends (FIG. 1e).

The design of the arrangement with stationary heating units betweenwhich hot air is distributed in accordance with a certain patternconstitutes the basis of a reliable operation and makes possible thechange-over of hot air during the different, individually very shortworking phases which have been described. The rapid change-over of theflow path of the hot air is also facilitated by the valve arrangementwhich can be moved simply and rapidly between the different, accurateworking positions.

The arrangement in accordance with the invention has been described witha heating set of the hot-air type which has proved to be the optimumsolution in the production of casings of laminated material comprisinglayers of foamed plastic. However, this does not mean that on workingwith a different sheet material the heating set cannot be designed withanother working medium. In certain cases, for example, a heating set ofthe infrared type may be appropriate.

Depending on whether the laminated material is symmetrical or not,whether it contains layers of different plastic material types ordepending on other factors, it may also be suitable to preheat the totalsheet, that is to say, preheat both sides of the sheet, which can bedone simply in accordance with the invention in that a part of the airintroduced to the preheating unit is conducted around the material sheetand is made to act also upon the opposite side of the same.

We claim:
 1. An arrangement for the manufacture of casings ofthermoplastic, heat-shrinkable material comprising forming tool meansfor the winding of a material sheet to casing-shape and means forjoining together ends of the material sheet in casing-shape, heating setmeans arranged along a feed track for the material sheet, said heatingset means having a preheating unit for the heating of the main length ofthe material sheet, and an intensive heating unit for the heating of afront end of the sheet facing towards the forming tool means, theheating set means being a hot air type and comprising a valve whichselectively conducts a hot air stream to the preheating unit and/or tothe intensive heating unit.
 2. The arrangement in accordance with claim1, wherein the intensive heating unit is arranged adjoining the formingtool means such that before the winding of the material sheet around theforming tool the side of the front end of the sheet facing towards thetool means is intensively heated and such that after the winding of thesheet the ends of the sheet are intensively heated to facilitateheat-sealing.
 3. The arrangement in accordance with claim 1 or 2,wherein the heating set means is stationary.
 4. The arrangement inaccordance with claim 1 or 2, wherein the preheating unit and theintensive heating unit each have a working width which substantiallycoincides with or slightly exceeds the corresponding width of thematerial sheet, the preheating unit and the intensive heating unithaving a combined working length corresponding to the length of thematerial sheet and the working length of the intensive heating unitbeing one third smaller than the working length of the preheating unit.5. The arrangement in accordance with claim 1 or 2, wherein theintensive heating unit is arranged between the preheating unit and theforming tool means.
 6. The arrangement in accordance with claim 1,wherein the valve has three working positions, a first position in whichthe supply of hot air to the heating units is interrupted, a secondposition in which hot air is supplied to both heating units and a thirdposition in which hot air is supplied to the intensive heating unit.